Most traditional ceramic manufactured products, such as wall tiles and floor tiles, are made of a ceramic body that confers form and mechanical properties to the object; the ceramic body generally has some porosity and poor aesthetic qualities.
Said ceramic body, which is defined “green” or, alternatively, “fired”, if previously fired, is then usually coated with a ceramic layer, called ceramic glaze; the ceramic glaze is completely sintered by firing, in such a way to gain suitable superficial aesthetic qualities and, in the meantime, to become a fluid-proof barrier; as a matter of fact, after firing, the ceramic glaze has usually no porosity and is generally resistant to abrasion and to the attack of chemical agents such as acids, bases, dyes.
The aesthetic finishing of the ceramic material can be completed by a decoration phase, that is by the application of sinterable and variously coloured ceramic materials (ceramic pigments) which are applied according to a precise drawing (décor).
The decor can be applied either on the green or fired ceramic body, on which the glaze was previously set, or, in the so called third firing decorations, after the firing, on the glaze.
Different techniques are used to transfer images to the ceramic substrate: i.e. screen printing and photogravure (commonly referred to as rotocolor). These technologies require flat substrate or with minimum roughness, they are suitable for mass production, but have very limited flexibility of new design set up and changeover between designs.
Another technique of printing decoration on ceramics is digital printing by inkjet technique.
Digital printing and decoration by inkjet technique is widely used in multiple sectors, such as graphic arts, textile industry, industrial marking and it is well known, both referring to printing equipments and also to the inks used.
Peculiarly in ceramic applications, the thermal treatment, which is required once the substrate has been printed, makes the conventional inks, that are used in the other applications and are mainly based on organic pigments, unsuitable for use.
Two kind of inks for inkjet printing of ceramics are known: inks constituted by solutions of metallic cations and inks based on dispersions of inorganic pigments.
Inorganic pigments used for traditional decoration in ceramic, such as screen printing and photogravure, have average particle size above 1 μm.
When inkjet inks based on dispersions of inorganic pigments are concerned, it is mandatory that the inorganic pigments are well dispersed in the liquid medium and possess nano-scale dimensions (i.e. below 1 μm in diameter), for the ceramic inkjet ink flows through small nozzles (30-100 μm in diameter) at high speed.
Nano-scale dimensions of the inorganic pigments are usually obtained by milling with microspheres the pigments, pre-dispersed in the medium, in the presence of a milling aid.
Examples of ceramic inkjet inks based on dispersions of inorganic pigments in polar organic mediums are described in EP 2159269, WO 2006/126189, EP 1840178; the inks are generically said to contain anti-settling and/or dispersing agents.
Nonetheless, there is still the industrial need for improved ceramic inkjet inks based on inorganic ceramic pigments having low viscosity, solid particle size below 0.8 □m, long shelf life and that can be printed on ceramic surfaces and passed through a high temperature kiln to form a permanently glazed print. The reaction products of polyamines and carboxyl terminated polyesters with formation of amidic and/or salt linkages are known products that belongs to a wide class of dispersants, which have been described in many patents; by way of example we cite: U.S. Pat. No. 4,224,212, U.S. Pat. No. 4,861,380, U.S. Pat. No. 5,700,395, U.S. Pat. No. 6,197,877 and U.S. Pat. No. 7,008,988. Such wide class of dispersants is generally suitable for use as dispersing agents for various solids in organic liquids. JP 2008184535 describes a reaction product of polyethylenimine with a co-polyester of caprolactone and lactic acid as dispersant mainly for the preparation of light filters.
Nonetheless, none of the above cited documents suggests that the reaction product of a polyethyleneimine and a homo- or co-polyester based on lactic acid may be suitable as adjuvant for the preparation and stabilization of inkjet inks for ceramic.
It has now been found that the reaction product of a polyethyleneimine and a homo- or co-polyester based on lactic acid can conveniently be used in the preparation of ceramic inkjet inks for inkjet printing machines.
Surprisingly, the reaction product of a polyethyleneimine and a co-polyester based on lactic acid, is perfectly suitable, in the milling phase, to fluidize the pre-dispersed inorganic pigments allowing their rapid milling and to subsequently prevent agglomeration and sedimentation of the nano-scale inorganic pigments in the final ink.